I found a similar question. But mine is regarding with Messaging Services. As you see the total data used is about 4.5GB. 4GB of them is used in recent two months. The Messaging Services is using about 2.4GB. I want to know why and how to disable Messaging Services because my phone is running in a private 4G network and we never use iMessage or Text Message etc.
First, you can check iMessage apps that appear in App tray in Messages (or look for a button with App Store logo beside the text field).
Are there any? If yes, you must check them.
Second, if you say that you "never use iMessage", you can turn it off in Settings.